天津市承压含水层条件下地下连续墙深度优化

对天津地区多个建筑基坑及地铁基坑观测发现,长时间大深度降水时,在较大的深度范围的土层内都存在越流现象,并且出现较大的地表沉降。基于对天津市市区各处基坑勘察报告的对比分析,利用天津市区典型工程地质条件,采用考虑降水井瞬态降水的三维有限差分数值模型进一步研究,根据突涌计算得到基坑底部水位降深,对不同地连墙深度条件下的5~25 m深基坑进行减压降水,得到渗流场结果后采用分层总和法计算基坑周边变形场。计算结果表明,降水开始后3~10 d内,水位趋于稳定,基坑外沉降在10~20 d发展较快;并且随着地连墙入土深度的增加,基坑外沉降减少;基于计算结果,提出针对不同环境安全等级条件下的地下连续墙深度优化方案。

Underground Diaphragm Wall Depth Optimization Considering the Confined Aquifer in Tianjin

Through long-term monitoring of excavations in Tianjin, it is clearly that the leakage between confined aquifers appears in great depth after a large scale drawdown in excavation. Meanwhile, obvious subsidence can also be detected around the excavation. Based on the geological survey investigations of urban district in Tianjin, the 3-D finite difference numerical model considering unsteady-state is used to analyze the phenomenon. To satisfy the safety coefficient of anti-uprush, the critical drawdown is calculated in different cases. It is noted that the range of depth of excavations in these cases is from 5 m to 25 m and the water levels of all these cases are lowered to the critical depth in the process of simulation. Based on the variation of seepage field, the subsidence in calculation region is obtained by using splitting summation method. The result indicates that the water level around excavation can stabilize after dewatering of 3 to 5 days. Besides that, the subsidence outside the excavation increases rapidly in the initial 10 to 20 days. Increasing with the depth of underground diaphragm wall, the subsidence around the excavation decrease. Furthermore, based on the calculated subsidence, optimization designs are proposed to meet the different safety requirement classes of surrounding environment.